Amyloid-β (Aβ) peptides are metabolites of the Alzheimer's disease-associated precursor protein, β-amyloid precursor protein (APP), and are believed to be the major pathological determinants of Alzheimer's disease (AD). These peptides consist mainly of 40 to 42 amino acids, Aβ1-40 (“Aβ40”) and Aβ1-42 (“Aβ42”), respectively. Aβ40 and Aβ42 are generated by two enzymatic cleavages occurring close to the C-terminus of APP. The enzymes responsible for the cleavage, β-secretase and γ-secretase, generate the—and C-termini of Aβ, respectively. The amino terminus of Aβ is formed by β-secretase cleavage between methionine residue 596 and aspartate residue 597 of APP (APP 695 isoform numbering) (see, e.g., U.S. Pat. No. 6,440,698; and U.S. Pat. No. 5,744,346).
γ-secretase activity cleaves at varying positions 38-, 40- or 43-residues C-terminal of this β-secretase cleavage to release Aβ peptides (for a review of γ-secretase and γ-secretase activity, see, e.g., U.S. Patent Application 20020025540). The complete molecular identity of γ-secretase enzyme is still unknown. Presenilin 1, or the closely related presenilin 2, is needed for γ-secretase activity. γ-secretase activity is reduced 80% in cultured cells derived from embryos genetically deleted for presenilin 1. All γ-secretase activity is lost in cells lacking both presenilin 1 and presenilin 2. Peptidomimetic inhibitors of γ-secretase activity can be crosslinked to presenilins 1 and 2, suggesting that these proteins are catalytic subunits for the cleavage. However, γ-secretase activity isolated from cells chromatographs as a large complex >1M daltons. Recent genetic studies have identified three more proteins required for γ-secretase activity; nicastrin, aph-1 and pen-1. (Francis et al., 2002, Developmental Cell 3(1): 85-97; Steiner et al., 2002, J. Biol. Chemistry: 277(42): 39062-39065; and Li et al., 2002, J. Neurochem. 82(6): 1540-1548). Accumulation of presenilin into high molecular weight complexes is altered in cells lacking these proteins.
A third enzyme, α-secretase, cleaves the precursor protein between the β- and γ-cleavage sites, thus precluding Aβ production and releasing an approximately 3 kDa peptide known as P3, which is non-pathological. Both β- and α-secretase cleavage also result in soluble, secreted—terminal fragments of APP, known as sAPPβ and sAPPα, respectively. The sAPPα fragment has been suggested to be neuroprotective.
In normal individuals, the Aβ peptide is found in two predominant forms, the majority Aβ-40 (also known as Aβ1-40) form and the minority Aβ42 (also known as Aβ1-42) form, each having a distinct COOH-terminus. The major histological lesions of AD are neuritic plaques and neurofibrillary tangles occurring in affected brain regions. Neuritic plaques consist of Aβ peptides, primarily Aβ40 and Aβ42. Although healthy neurons produce at least ten times more Aβ40 compared to Aβ42, plaques contain a larger proportion of the less soluble Aβ42. Patients with the most common form of familial Alzheimer's disease show an increase in the amount of the Aβ42 form. The Aβ40 form is not associated with early deposits of amyloid plaques. In contrast, the Aβ42 form accumulates early and predominantly in the parenchymal plaques and there is strong evidence that Aβ42 plays a major role in amyloid plaque deposits in familial Alzheimer's disease patients (Roher et al., 1993, Proc. Natl. Acad. Sci. USA 90:10836; Iwatasubo, T., et al., 1994 Neuron 13:45; Yamaguchi et al., 1995, Amyloid Int. J. Clin. Invest. 2:7-16; and Mann et al., 1996 Am. J. Pathol. 148:1257).
Neurofibrillary tangles consist of aggregated tau protein and their role in AD pathology is less clear. AD symptoms are most closely correlated with total brain Aβ rather than plaques. About 10% of AD cases result from autosomal dominant inheritance of mutations in either the APP or the presenilin 1 and presenilin 2 genes. In both cases, increased production of total Aβ or Aβ42 versus Aβ40 results. There is strong evidence that Aβ peptides are critical in the pathogenesis of Alzheimer's disease. Therefore, compositions which modulate the activity and/or levels of these peptides are desired.
For example, U.S. Patent Application 20020128319 A1 states that certain non-steroidal anti-inflammatory drugs (NSAIDS) lower production and/or levels of Aβ42 in cell cultures expressing Aβ40 and Aβ42 derived from the cleavage of APP. Since there is good evidence that high Aβ42 levels are a major risk factor for AD, such drugs may be useful in preventing, delaying or reversing the progression of AD. The drawback of the use of such drugs, however, is that large doses of NSAIDS are required for significant lowering of Aβ42, and significant gastrointestinal side effects, including bleeding ulcers, are associated with prolonged use of NSAIDS at high doses (Langman et al., 1994, Lancet 343: 1075-1078). In addition, there remains an unknown risk for Alzheimer's disease due to amyloid formation from Aβ40 and other forms unaffected by Aβ42 lowering agents.
There is, therefore, a need in the art to develop treatments for diseases or disorders related to the regulation of Aβ production. Furthermore, there is a need to develop methods of screening that can be used to identify compounds to treat such diseases or disorders.
Citation or identification of any reference in Section 2, or in any other section of this application, shall not be considered an admission that such reference is available as prior art to the present invention.